1. Field of the Invention
The present invention relates to an exposure apparatus, more particularly, to an automatic focus control device used in an exposure apparatus in the production of an integrated circuit.
2. Description of the Prior Art
Recent research and development projects in the field of integrated circuits (IC's) and large-scale integrated circuits (LSI's) have concentrated on the achievement of greater miniaturization of the elements and higher integration of the circuit patterns. In the production of these IC's, the so-called stepper exposure apparatus, equipped with a reduction lens, is used to expose a wafer. In this process, the focal depth of the reduction lens is usually in the range of about 1 .mu.m. However, the surface of the wafer or workpiece usually has several bumps or depressions, and the depth of these bumps or depressions is usually in the range of from 5 .mu.m as a minimum value to 10 .mu.m as a maximum value. For this reason, it is necessary to repeatedly adjust the focus of the reduction lens to correspond to the height of the surface of the wafer by an automatic adjustment. In order to automatically adjust the focus, it is necessary to precisely measure the height of the reduction lens from the wafer to be exposed.
A well-known technique for measuring the height of the wafer to be exposed is that of using an air-micrometer or of utilizing electrostatic capacity. In the case of the air-micrometer, the air used for measuring the height cannot be blown against the center portion of the exposing portion because the light beam used to expose the wafer is located at that portion. Therefore, in order to measure the height of the portion of the wafer to be exposed, the air used for measuring the height is blown against a peripheral portion of the wafer. The height of the center portion of the exposing portion is then presumed from the values obtained when measuring the height of the peripheral portion. When electrostatic capacity is used, as with the air-micrometer, a sensor for measuring the electrostatic capacity cannot be provided on the center portion of the exposing portion. The height of the center portion of the exposing portion, therefore, is similarly presumed from the values obtained when measuring the height of the peripheral portion.
Further, when one of the chips located near the edge of the wafer on an X-Y table is exposed by the light beam the surface of the wafer is often damaged by the descent of the reduction lens because one air-micrometer or electrostatic capacity sensor is provided to the wafer side and the other air-micrometer or electrostatic capacity sensor is provided to the X-Y table and the air-micrometer or the sensor provided to the X-Y table side cannot detect the height of the chips to be exposed.
In one attempt to solve the above-mentioned problems, a method for detecting the height of the wafer using reflected rays, i.e., the so-called focus detecting method utilizing reflected rays, is used. However, problems also occur when this method is used, as explained hereinafter.